A liquid crystal display is different from other types of displays, such as a CRT (Cathode Ray Tube), a PDP (Plasma Display Panel), and an EL (Electro Luminescence) in that it displays characters and images not by allowing liquid crystal to self-emit light, but by adjusting and controlling a quantity of transmitting light emitted from a particular light source or a quantity of reflected light.
The foregoing conventional liquid crystal display is largely divided into two types: a transmitting liquid crystal display and a reflecting liquid crystal display.
The transmitting liquid crystal display device has polarizing plates at the light incident side and light outgoing side, and displays an image by modulating in the liquid crystal layer a polarization state of linearly polarized light incident through the polarizing plate at the light incident side, and controlling a quantity of light passing through the other polarizing plate at the light outgoing side. Hence, a light emitting source, such as a fluorescent tube or an EL, serving as lighting means and called a back light that lights the liquid crystal display from behind (light incident side), is generally provided to the transmitting liquid crystal display at the light incident side.
On the other hand, the reflecting liquid crystal display includes one polarizing plate and one reflecting plate, and it controls a quantity of outgoing light from the polarizing plate by modulating a polarization state of linearly polarized light in the liquid crystal layer while the linearly polarized light incident on the polarizing plate is reflected by the reflecting plate to reach the polarizing plate again. Hence, the reflecting liquid crystal display can display an image by using surrounding light, and does not require the back light as described above, thereby making it possible to realize the characteristics of the transmitting liquid crystal display, that is, lightness, thinness, and a low power consumption.
Further, under the very bright circumstances with direct sunlight, image visibility is reduced markedly in a light-emitting display or the transmitting liquid crystal display. On the contrary, another characteristic of the reflecting liquid crystal display is that the image visibility can increase under such circumstances.
For this reason, there has been an increasing need for the reflecting liquid crystal displays in recent years, and there is a tendency to employ reflecting liquid crystal displays extensively in portable information terminals, mobile computers, etc.
However, the foregoing reflecting liquid crystal display has the following problem. That is, as previously mentioned, because the reflecting liquid crystal display uses surrounding light for display, its display luminance largely depends on environments, and one can hardly see the display under dark circumstances at night, for example. In particular, this problem is significant in a reflecting liquid crystal display using a color filter for color image display or a polarizing plate. Therefore, if the surrounding light is insufficient, auxiliary lighting means has to be provided.
However, the back light used in the transmitting liquid crystal display can not be employed as the lighting means for the reflecting liquid crystal display, because a reflecting plate made of a metal thin film or the like is essentially provided at the back surface of the reflecting liquid crystal display.
In order to solve the above problem, methods of lighting the reflecting liquid crystal display from front have been proposed as auxiliary lighting means under the dark environment.
It should be noted, however, that, in order to maintain the advantages of the reflecting liquid crystal display, namely, the lightness, thinness, and low power consumption, the auxiliary lighting means has to be a light, low-power consuming, space-saving component.
For example, if a fluorescent tube is used as the light source of the auxiliary lighting means, a high frequency power is necessary to allow the fluorescent tube to emit light. However, because a portable device is equipped with a DC power source, such as a battery, an inverter that converts a DC current to an AC current is essential. Hence, not only power consumption increases, but also a larger space becomes necessary to install the high frequency power source, thereby making it difficult to produce a low-power consuming, light, compact reflecting liquid crystal display.
However, by using a light source which can emit light on a DC current, such as an LED, as the light source of the auxiliary lighting means, the inverter can be omitted. Thus, this arrangement is advantageous in realizing a low-power consuming, light, space-saving reflecting liquid crystal display.
Lighting systems described in Japanese Laid-Open Patent Application No. 260405/1998 (Japanese Official Gazette, Tokukaihei No. 10-260405, published on Sep. 29, 1998), U.S. Pat. No. 5,506,929 issued to Ping-Kaung Tai Clio Technologies Inc. and published on Apr. 9, 1996, etc. can be used as the foregoing auxiliary lighting means. These publications disclose a lighting system, in which a light guiding body is combined with converting means for efficiently converting light emitted from a dot light source to light in a linearly emitting state.
Also, U.S. Pat. No. 5,608,550 issued on Mar. 4, 1997 discloses an auxiliary lighting system, by which light emitted from a dot light emitting source is converted efficiently to light in a linearly emitting state in a uniform distribution, and then this light in the linearly emitting state is guided to reach a planar light guiding body to have a tilt with respect to the incident surface thereof, whereby the light in the linearly emitting state is converted to light in a planarly emitting state.
The above lighting system can reduce the number of light sources compared with a case where a plurality of dot light emitting sources are provided at the end surface of the light guiding body, and is advantageous in that it costs less. The above lighting system would also be advantageous in that a light-dark difference in luminance can be reduced compared with a case when a plurality of dot light emitting sources are provided at the end surface of the light guiding body.
The following will describe the conditions required for the lighting system employed in a liquid crystal display. The liquid crystal display displays an image by lighting a liquid crystal element, and for this reason, it is preferable to light the pixel area (display screen) formed on the liquid crystal element brightly in a uniform manner. In order to realize such lighting, a dot of light emitted from a dot light source has first to be converted to light in a linearly emitting state efficiently in a uniform distribution. In other words, the light converted to the linearly emitting state is allowed to have, in the light emitting state, the least irregularity in luminance with respect to the line direction, while attaining high outgoing luminance by efficiently utilizing light emitted from the dot light source.
In particular, when using the auxiliary lighting means as a front light, light in the linearly emitting state has to be converted to light in the planarly emitting state, and the resulting planarly emitting light should not affect the display quality adversely. For example, a moire fringe is produced when a structure formed in the planar light guiding body that converts light in the linearly emitting state to light in the planarly emitting state interferes with a pixel formed on the liquid crystal display element, and the moire fringe gives adverse affects significantly to the display quality. Thus, the occurrence of moire fringe has to be prevented.
To this end, it is preferable that the linear light guiding body converts a dot of light incident thereon to light in the linearly emitting state adequately as light to be incident on the planar light guiding body by considering the position of the planar light guiding body in relation to the liquid crystal element. Also, it is important to convert a dot of light to light in the linearly emitting state efficiently in a well-distributed state, and then to light in the planarly emitting state.
However, the foregoing auxiliary lighting means has the following problem.
First, the lighting system described in Japanese Laid-open Patent Application No. 260405/1998 supra discloses that light from a dot light emitting source converted to light in the linearly emitting state to reach the planar light guiding body. However, this publication fails to disclose or suggest specifically the linearly emitting state (for example, as to the outgoing direction or distribution). The same can be the for U.S. Pat. No. 5,506,929 supra, which will be described more in detail below.
FIG. 42(a) shows an example of the lighting system described in U.S. Pat. No. 5,506,929 supra.
As shown in the drawing, the lighting system includes a light guiding body 2102 in the vicinity of a dot light emitting source 2101. The light guiding body 2102 guides incident light thereon to another light guiding body 2104 by limiting an angle of the incident light by a propagating portion 2102b, and then allowing the same to go out from a light outgoing surface 2012c to reach the light guiding body 2104. As a consequence, the incident light is converted to light in the planarly emitting state by the light conducting body 2104, thereby making it possible to light an unillustrated reflecting display.
In the lighting system arranged as above, however, the length of the light outgoing surface 2102c of the light guiding body 2102 is substantially equal to an effective emission length of a light incident surface 2104a of the light guiding body 2104. Therefore, sufficient light does not enter at the corners of the light incident surface 2104a of the light guiding body 2104, and this produces shadows 2103 noticeable as shown in FIG. 42(b) when one observes the display screen, thereby causing a problem that the display quality its deteriorated.
While the auxiliary light stays OFF, cyclic structures 2104f formed on the light guiding body 2104 interfere with a repeating direction (not shown) of pixels formed in the reflecting liquid crystal display, and a moire fringe is produced, which also causes a problem that the display quality is deteriorated.
Further, when a plurality of dot light emitting sources 2101 are provided directly to the light incident surface 2104a of the light guiding body 2104 in the foregoing lighting system, incident light emitted from each dot light emitting source 2101 directly travels through the light guiding body 2104. Thus, bright lines as many as the dot light emitting sources 2101 are produced, thereby causing a problem that a light-dark difference in luminance is produced on the display screen and the display quality is deteriorated markedly.
Also, when the lighting system described in U.S. Pat. No. 5,608,550 supra displays an image while the front light stays OFF, that is, by using surrounding light alone (light incident from the exterior in all directions), an image is blurred or a moire fringe is produced, thereby degrading the display quality. In addition, in the above lighting system, a tilt is given to light incident on the planar light guiding body with respect to the surface thereof. Thus, light other than the one that goes out in the planar light guiding body direction is not utilized, thereby reducing light utilization.
Further, in the above lighting system, when more than one dot light emitting source is provided in order to increase brightness, for example, when a dot light emitting source is provided at each end of the linear light guiding body, light emitted from each dot light emitting source is converted into light in the nonuniform linearly emitting state. This inconvenience may be eliminated and a well distributed linearly emitting state may be obtained by providing two linear light guiding bodies disclosed in the publications or the like and symmetrically with respect to each other. However, this arrangement upsizes the light source unit, and not only the portability is reduced, but also the cost is increased significantly.
Moreover, none of the foregoing publications fully discusses light utilization of the linear light guiding body. Thus, a great quantity of light is lost at the linear light guiding body, thereby reducing light utilization of the entire lighting system.
More specifically, according to each technique disclosed in the foregoing publications, it is quite difficult to secure a sufficiently large reflecting surface which plays an important role in converting light to light in the linearly emitting state while distributing the same in a uniform manner. Therefore, a quantity of light that goes out in an adequate direction from the linear light guiding body is reduced and so is a quantity of light contributing in planarly emission in the planar light guiding body, thereby reducing light utilization.
As has been discussed, there has been a need for bright, inexpensive, low-power consuming lighting system and liquid crystal display which do not cause shadows or a moire fringe from the light source, or a light-dark difference in luminance.